Recently, there is an increasing demand for a secondary battery or the like, which is used in PDA, a portable electronic device, a home-use compact power storage facility, an auto-bicycle powered by a motor, an electric vehicle, a hybrid electric vehicle or the like. In particular, there is a growing demand for a lithium-ion battery as a secondary battery with a high capacity.
Conventional lithium-ion batteries involve a concern of liquid leakage or danger of ignition, since it uses an electrolysis solution containing an organic solvent as an electrolyte. Therefore, a lithium-ion battery obtained by using a solid electrolyte that contains no solvent has made studied.
As a solid electrolyte, Patent Document 1 discloses a sulfur-based solid electrolyte, for example. This solid electrolyte has a high ion conductivity of 10−3 S/cm or more. The solid electrolyte is used not only in an electrolyte layer of a lithium-ion battery, but also in a positive electrode layer or a negative electrode layer in the mixture of an active material or the like.
Meanwhile, an all-solid battery used in an electric vehicle is required to have high performances such as a high capacity and a high output power.
As a positive electrode mix, a mixture of a sulfur-based solid electrolyte disclosed in Patent Document 1 mentioned above and an oxide-based positive electrode active material can be given, for example.
However, the oxide-based positive electrode active material does not have a high capacity. Therefore, even if it is possible to allow it to have a high output power, allowing it to have a high capacity has limitations.
For allowing it to have a high capacity, it is advantageous to use a sulfur-based active material, which has a large theoretical capacity. In Patent Document 2, a positive electrode material is prepared by mixing thio-lisicon Li3.25Ge0.25P0.75S4, that is one of sulfur-based solid electrolytes using sulfur as a positive electrode active material, and an all-solid lithium battery using the thio-lisicon Li3.25Ge0.25P0.75S4 as an electrolyte is evaluated for performance.
However, it cannot be admitted that it fully utilizes the capacity of sulfur. In addition, it has a disadvantage that the internal resistance is high and the discharge voltage is low. Another disadvantage is that the production cost is high due to the high cost of Ge.